1. Title of the invention
Determination of lay-up sequence and detection of the missing ply in cured fiber reinforced composite using optical microscope

2. Field of invention
This invention is related to Manufacturing and Quality Assurance of Fiber Reinforced composite materials.

3. Use of invention
This invention is used to determine with reliability the layup sequence in the cure fiber reinforced composite component. Thus it is useful to detect any missing plies in the component due to manual manufacturing errors and hence an effective quality assurance tool incase of dispute or doubt. Further, this technique can be used to determine the lay up sequence of cured laminate whose lay up details are not known.

4. Prior art
Normally, the layup sequence in manual layup of composite components is ensured by in-process inspection during lay up stage. The correctness of layup is indirectly ensured by the thickness and mass measurements. However, in-process inspection is found to be unreliable due to human errors, especially in missing of partial layers as in the case of monolithic zones of honeycomb sandwich components.

5. Draw backs of prior art
The following are the drawbacks:
• The method is-unreliable and prone to errors and is subjective.
• The thickness of fiber reinforced composite components has variation of ±7% at the best controls.
• The mass of the components has acceptable variation of ±5%
• fartial acid digestion was also attempted which involves environmental and health hazards. -
Hence, in moderately thick components it is very difficult to ascertain the presence of all the required layers in the component which otherwise detrimental effect on the performance of composite components.

6. Comparison between prior art and present invention
The prior art based on thickness and mass is subjective and missing- layers can go undetected in moderately thick components. The partial acid digestion is cumbersome and involves handling hazardous chemicals. The present invention is simple, reliable and needs a small sample, polishing and an optical microscope of magnification X 50 and above preferably with table movement to measure the layer thickness.

7. Aim of the invention
The aim of the invention is to determine the layup sequence in a given cured laminated fiber reinforced composite laminate. This in turn can be used to detect the missing plies or wrongly located plies and hence an effective quality control tool.

8. Summary of the present invention
The present invention needs a sample minimum 15 mm X 15 mm with one of the major direction marked on the specimen, with reference to which other directions are determined. The specimen is cut to the pentagonal shape as detailed in the next section, and wet polished with fine emery paper of in the polishing machine. The specimen is viewed in four directions (In aerospace composite structural design n/4 family layup is universally adopted) to accurately predict the number of layers, and layup sequence.

9. Brief description of drawings
Figure 1. Typical layup sequence and common method of preparing fiber reinforced claminated composite.
Figure 2. Schematic of the specimen suggested in this invention.
Figure 3. Typical appearance of polished surface at primary angles and Determination of layup sequence

10. Statement of invention
This invention is to determine the layup sequence of continuous fiber reinforced laminated composite.

11. Detailed description of invention
Continuous fiber reinforced laminated composites are fabricated using thin plies called prepregs. This prepregs are laid up as per the design requirement over a layup tool and vacuum bagged and finally curd in the autoclave. In aircraft industry the n/4 family of lay up is being followed almost universally. Figure 1 schematically represents a laminate and its constituent layers.

The figure 2 shows the schematic view of the specimen suggested in this invention. The area of dispute over the missing plies or erroneous layup is chosen with one known direction marked on the specimen. Then the specimen is cut to the shape as shown in the figure, such that there are two pair perpendicular surfaces. Thus the first pair is parallel to 0 & 90 and the other pair is parallel to +45 and -45 plies in the laminate.

The surfaces are polished and viewed through microscope, in the example taken for explanation a four layer laminate is considered. The typical appearance of a ply parallel to the plane of observation is long continues bright streaks. The layer perpendicular to the plan of observation appears as dense dark dots as schematically shown in the figure 3. The layers at the inclination of +45 and -45 degrees to the plane of observation appear as dense, slightly elongated dots. It is difficult to differentiate between the plies oriented at 90,+45 and -45 degrees to the plane of observation. The four plane of observation suggested in this invention helps to identify the for groups of layers without any ambiguity hence become very reliable Quality assurance tool in case of dispute or for verification of layers in more critical zones.

Claims
Fiber reinforced composite materials are used in manufacturing of aircraft structural components. Most of the time, prepreg layers are manual laid up in the desired orientation and sequence. The errors in orientation & sequence, missing plies and mis-located plies are quite likely to occur in manual lay up process. Such errors have detrimental effect on the performance of the components, are very difficult to detect once the component is cured in the autoclave. In this work the following claims are made.

1. A simple methodology to verify and find out the missing layers and layup sequence in the cured composite components, made of Uni-directional and fabric reinforcements either with prepregs or with wet-layup.

2. This invention claims to reliable determination of layup sequence of any given cured laminate when, no manufacturing details are available as in the case of field repair of damaged composite components.

3. In this method trimmed edges from the suspected area or critical area of the components is used to establish the sequence and orientation of the plies.

4. This method with the help of horoscope the layup sequence and hence missing layers at critical joints or design critical zones, where hole has been drilled or cur-out has been made.

5. This invention further claims accurate assessment of cured ply thickness when travelling microscope is used.
.
6. Simple specimen design, along with optical microscope and highly reliable technique is
used in this invention.